Russia should not allow Syrian dictator Bashar Assad to launch a “reckless escalation” of violence in rebel-held territory, a top U.S. diplomat said Thursday.

Ambassador Nikki Haley, the U.S. representative to the United Nations, reiterated Western warnings that Assad must not use chemical weapons in his expected offensive against Idlib, one of the last areas controlled by opposition groups in northwest Syria. But she called for the regime to scrap plans even for a conventional attack on the provincial city.